1. Field of the Invention
The present invention generally relates to wireless communication systems and, more particularly, to the transmission and reception of physical downlink control channels.
2. Description of the Related Art
A communication system includes a DownLink (DL) that conveys transmission signals from Transmission Points (TPs) such as Base Stations (BS or NodeBs) to User Equipments (UEs), and an UpLink (UL) that conveys transmission signals from UEs to Reception Points (RPs) such as NodeBs. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as an access point or some other equivalent terminology.
DL signals include data signals conveying information content, control signals conveying DL Control Information (DCI), and Reference Signals (RS) that are also known as pilot signals. A TP transmits data information or DCI to UEs through respective Physical DL Shared CHannels (PDSCHs) or DL Control CHannels (CCHs). UL signals also consist of data signals, control signals, and RS. A UE transmits data information or UL Control Information (UCI) to an RP through a respective Physical Uplink Shared CHannel (PUSCH) or a Physical Uplink Control CHannel (PUCCH).
A PDSCH transmission to a UE or a PUSCH transmission from a UE may be in response to dynamic scheduling or to Semi-Persistent Scheduling (SPS). With dynamic scheduling, a TP conveys to a UE a DCI format, through a respective Physical DL Control CHannel (PDCCH), that provides a Scheduling Assignment (SA) for a PDSCH (DL SA) or for a PUSCH (UL SA). With SPS, a PDSCH or a PUSCH transmission is configured to a UE by a TP through higher layer signaling, such as Radio Resource Control (RRC) signaling, to occur at predetermined time instances and with predetermined parameters as informed by the higher layer signaling.
A TP may also transmit multiple types of RSs including a UE-Common RS (CRS), a Channel State Information RS (CSI-RS), and a DeModulation RS (DMRS). A CRS is transmitted over substantially an entire DL system BandWidth (BW) and can be used by all UEs to demodulate data or control signals, or to perform measurements. To reduce an overhead associated with a CRS, a TP can transmit a CSI-RS with a smaller density in a time domain and/or frequency domain than a CRS for UEs to perform measurements. A TP can transmit a DMRS only in a BW of a respective PDSCH. A UE may use a DMRS to demodulate information in a PDSCH.
FIG. 1 illustrates a transmission structure for a DL Transmission Time Interval (TTI).
Referring to FIG. 1, a DL TTI includes one subframe 110 which includes two slots 120 and a total of NsymbDL symbols for transmitting data information, DCI, or RS. A first number of MsymbDL symbols are used to transmit PDCCHs and other control channels (not shown) 130 and can be informed to a UE either through a transmission of a Control channel Format Indicator (CFI) field in a Physical Control Format Indicator CHannel (PCFICH) or through higher layer signaling such as Radio Resource Control (RRC) signaling. A remaining number of NsymbDL−MsymbDL symbols are primarily used to transmit PDSCHs 140. A transmission BW consists of frequency resource units referred to as Resource Blocks (RBs). Each RB consists of NscRB sub-carriers, or Resource Elements (REs), and a UE is allocated MPDSCH RBs for a total of MscPDSCH=MPDSCH·NscRB REs for a PDSCH transmission BW. An allocation of one RB in a frequency domain and of one slot or two slots (one subframe) in a time domain will be referred to as a Physical RB (PRB) or as a PRB pair, respectively. Some REs in some symbols contain CRS 150, CSI-RS or DMRS.
DCI can serve several purposes. A DCI format in a respective PDCCH may schedule a PDSCH or a PUSCH providing data or control information to or from a UE, respectively. Another DCI format in a respective PDCCH may schedule a PDSCH providing System Information (SI) to a group of UEs for network configuration parameters, or a response to a Random Access (RA) by UEs, or paging information, and so on. Another DCI format may provide to a group of UEs Transmission Power Control (TPC) commands for SPS transmissions in respective PUSCHs or PUCCHs.
A DCI format includes Cyclic Redundancy Check (CRC) bits in order for a UE to confirm a correct detection. The DCI format type is identified by a Radio Network Temporary Identifier (RNTI) that scrambles the CRC bits. For a DCI format scheduling a PDSCH or a PUSCH to a single UE, the RNTI is a Cell RNTI (C-RNTI). For a DCI format scheduling a PDSCH conveying SI to a group of UEs, the RNTI is an SI-RNTI. For a DCI format scheduling a PDSCH providing a response to an RA from a group of UEs, the RNTI is an RA-RNTI. For a DCI format scheduling a PDSCH paging a group of UEs, the RNTI is a P-RNTI. For a DCI format providing TPC commands to a group of UEs, the RNTI is a TPC-RNTI. Each RNTI type is configured to a UE through higher layer signaling from a TP (and the C-RNTI is unique for each UE).
FIG. 2 illustrates an encoding and transmission process for a DCI format.
Referring to FIG. 2, an RNTI of a DCI format masks a CRC of a codeword in order to enable a UE to identify a DCI format type. A CRC 220 of (non-coded) DCI format bits 210 is computed and it is subsequently masked at 230 using an exclusive OR (XOR) operation between CRC 220 and RNTI bits 240; that is, XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. A masked CRC is then appended to DCI format bits 250, channel coding 260 is performed, for example using a convolutional code, followed by rate matching 270 to allocated resources, and finally by interleaving and modulation 280, and transmission of a control signal 290. For example, both a CRC and an RNTI consist of 16 bits.
FIG. 3 illustrates a reception and decoding process for a DCI format.
Referring to FIG. 3, a received control signal 310 is demodulated and resulting bits are de-interleaved at the demodulator & de-interleaver 320, a rate matching applied at a TP is restored at a rate de-matcher 330, and control information is subsequently decoded a channel decoder 340. After decoding, DCI format bits 360 are obtained after extracting CRC bits at the CRC extraction unit 350 which are then de-masked 370 through a XOR operation with an RNTI 380. Finally, a UE performs a CRC test at the CRC tester 390. If the CRC test passes, a UE considers a respective DCI format as valid and determines parameters for PDSCH reception or PUSCH transmission. If the CRC test does not pass, the UE disregards a respective DCI format.
A TP separately codes and transmits a DCI format in a respective PDCCH. To avoid a first PDCCH transmission blocking a second PDCCH transmission, a location of each PDCCH in a DL control region is not unique. As a consequence, a UE needs to perform multiple decoding operations per subframe to determine whether there is a PDCCH intended for it. REs carrying a PDCCH are grouped into Control Channel Elements (CCEs) in a logical domain. CCE aggregation levels may consist, for example, of 1, 2, 4, and 8 CCEs. Each CCE of a PDCCH is formed by Resource Element Groups (REGs) consisting of REs which are interleaved over a PDCCH transmission BW. For example, a CCE may consist of nine REGs which are formed by four consecutive REs (after discounting REs allocated to other signals).
For a PDCCH detection, a UE may determine a search space for candidate PDCCH locations after it restores CCEs in a logical domain according to a UE-common set of CCEs (Common Search Space or CSS) and according to a UE-dedicated set of CCEs (UE-Dedicated Search Space or UE-DSS). A CSS may consist of first C CCEs in a logical domain which may be used to transmit PDCCHs for DCI formats associated with UE-common control information and use an SI-RNTI, a P-RNTI, a TPC-RNTI, and so on, to scramble respective CRCs. A UE-DSS consists of remaining CCEs which are used to transmit PDCCHs for DCI formats associated with UE-specific control information and use respective C-RNTIs to scramble respective CRCs. CCEs of a UE-DSS may be determined according to a pseudo-random function having as inputs UE-common parameters, such as a subframe number or a total number of CCEs in a subframe, and UE-specific parameters such as a C-RNTI. For example, for a CCE aggregation level of Lε{1, 2, 4, 8} CCEs, CCEs for PDCCH candidate m are given by Equation (1):CCEs for PDCCH candidate m=L·{(Yk+m)mod └NCCE,k/L┘}+i  (1)
In Equation (1), NCCE,k is a total number of CCEs in subframe k, i=0, . . . , L−1, m=0, . . . , MC(L)−1, MC(L) is a number of PDCCH candidates for aggregation level L, and “mod” is the modulo operation. For example, for Lε{1, 2, 4, 8}, MC(L)ε{6, 6, 2, 2}. For UE-DSS, Yk=(A·Yk-1)mod D where Y−1=C−RNTI≠0, A=39827 and D=65537. For CSS, Yk=0.
A conventional DL control region may occupy a maximum of MsymbDL=3 subframe symbols and a PDCCH is transmitted substantially over an entire DL BW. Consequently, network functionalities that are needed in several operating cases, such as extended PDCCH capacity and PDCCH interference coordination in a frequency domain cannot be supported. One such case is the use of Remote Radio Heads (RRHs) in a network where a UE may receive DL signals either from a macro-TP or from an RRH. If RRHs and a macro-NodeB share a same cell identity, cell splitting gains do not exist and expanded PDCCH capacity is needed to accommodate PDCCH transmissions from both a macro-TP and RRHs. Another case is for heterogeneous networks where DL signals from a pico-TP experience strong interference from DL signals from a macro-TP, and interference coordination in a frequency domain among TPs is needed.
A direct extension of a conventional DL control region size to more than MsymbDL=3 subframe symbols is not possible at least due to a need for support of conventional UEs which cannot be aware of or support such extension. An alternative is to support DL control signaling in a PDSCH region by using individual PRB pairs. A PDCCH transmitted in one or more PRB pairs of a conventional PDSCH region will be referred to as Enhanced PDCCH (EPDCCH).
FIG. 4 is a diagram illustrating an EPDCCH transmission structure in a DL TTI.
Referring to FIG. 4, EPDCCH transmissions start immediately after PDCCH transmissions 410 and are transmitted over all remaining DL subframe symbols. EPDCCH transmissions may occur in four PRB pairs, 420, 430, 440, and 450, while remaining PRB pairs 460, 462, 464, 466, and 468 may be used for PDSCH transmissions. PRB pairs may be grouped in PRB Groups (RBGs) and a PDSCH transmission may be in RBGs. A UE may not receive PDSCH in a PRB pair within an RBG if this PRB pair is used to transmit the EPDCCH scheduling the PDSCH. As an EPDCCH transmission over a given number of subframe symbols may require fewer REs than the number of subframe symbols available in a PRB pair, multiple EPDCCHs may be multiplexed in a same PRB pair. The multiplexing can be in any combination of possible domains (i.e., time domain, frequency domain, or spatial domain) and, in a manner similar to a PDCCH, an EPDCCH includes at least one Enhanced CCE (ECCE).
A UE can be configured by higher layer signaling from a TP as one or more sets of PRB pairs that may convey EPDCCHs. The transmission of an EPDCCH to a UE may be in a single PRB pair (or a few PRB pairs), if a TP has accurate CSI for the UE and can perform Frequency Domain Scheduling (FDS) or beam-forming, or it may be in multiple PRB pairs, possibly also using transmitter antenna diversity, if accurate CSI per PRB pair is not available at a TP. An EPDCCH transmission over a single PRB pair (or few PRB pairs) will be referred to as localized or non-interleaved while an EPDCCH transmission over multiple PRB pairs will be referred to as distributed or interleaved.
An exact search space design for EPDCCHs is not material to the present invention and may or may not follow same principles as for PDCCHs. An EPDCCH consists of respective Enhanced CCEs (ECCEs) and a number of EPDCCH candidates exist for each possible ECCE aggregation level LE, for example, LEε{1, 2, 4, 8} ECCEs for localized EPDCCHs and LEε{1, 2, 4, 8, 16} ECCEs for distributed EPDCCHs. An ECCE may or may not have a same size as a CCE and, similar to a CCE, it is formed by Enhanced REGs (EREGs). For example, an ECCE may consist of four EREGs that are formed by nine consecutive REs that, unlike REGs, include REs that may be allocated to transmissions of other signals. EREGs do not include REs allocated to DMRS that are assumed to always be present in a PRB pair configured for EPDCCH transmissions in order for a UE to perform demodulation. Each EREG is contained within a PRB pair. For localized EPDCCH, all EREGs of an ECCE may be in a same PRB pair. For distributed EPDCCH, EREGs of an ECCE are distributed in different PRB pairs.
FIG. 5 illustrates EREG and ECCE structures for a distributed EPDCCH.
Referring to FIG. 5, a PRB pair consists of 12 REs and 14 subframe symbols 510. Excluding only REs allocated to DMRS transmissions in a PRB pair 520, 522, there are 144 remaining REs which are divided among sixteen EREGs 530 with each EREG consisting of nine REs 532 which are sequentially mapped across EREGs in a PRB pair first in a frequency domain and then in a time domain across a subframe symbols. For example, REs for EREG 10 are labeled by 10 540 and REs for EREG 40 are labeled by 14 542. For a distributed EPDCCH transmission over four PRB pairs 550, 552, 554 and 556, each PRB pair contains a respective EREG for an ECCE of a distributed EPDCCH for a total of 16 ECCEs 560. For example, ECCE#4 570 consists of EREG#4 in PRB#0, EREG#8 in PRB#1, EREG#12 in PRB#2, and EREG#0 in PRB#3.
A reason for assigning different EREGs in different PRB pairs to a same ECCE is for improved equivalence of actual sizes among ECCEs since, although each EREG nominally consists of 9 REs (and each ECCE of 36 REs), not all REs can be used for transmitting an EPDCCH as some REs may be used for transmitting other signals or channels such as CRS, CSI-RS, or PDCCH. Mixing EREGs forming an ECCE averages a discrepancy in a number of useful REs in each EREG. Cycling (with wrap around) EREGs in each PRB pair by 4 EREGs relative to a previous PRB pair, as illustrated in FIG. 5, typically improves an equivalence of actual sizes among ECCEs. This determination of an EREG index may be described by Equation (2) as:EREG index=(k+4·i)mod 16  (2)
In Equation (2), k=0, 1, . . . ,15 is an ECCE index and i=0, 1, 2, 3 is an index of a PRB pair containing a respective EREG.
A number of PRB pairs for transmitting EPDCCH in a subframe may depend on a DL operating BW which may range from 6 RBs to 100 RBs. For a maximum of about ⅓ of total PRB pairs in a DL operating BW allocated to EPDCCH transmissions, a number of 2, {2, 4, 6}, and {4, 6, 8} PRB pairs, respectively, can be used to transmit EPDCCHs for operating BWs of 6, 15, and 25 PRB pairs. For larger operating BWs consisting of 50 or more RBs, a number of PRB pairs allocated to EPDCCH transmissions can be assumed to be a multiple of four.
A symmetry resulting from having four PRB pairs for EPDCCH transmissions, with each PRB pair containing one EREG for one ECCE, does not hold if a number of PRB pairs is not an integer multiple of four. A new mapping of EREGs in each PRB pair is then required subject to a condition that for a distributed EPDCCH transmission a PRB pair contains only one EREG for a respective ECCE in order to optimally exploit a frequency diversity and an interference diversity by distributing EREGs of an EPDCCH in different PRB pairs.
Moreover, when a number of PRB pairs allocated to EPDCCH transmission in a subframe is a multiple of four, an arrangement of respective multiple sets of 4 PRB pairs should be defined taking into account a need to equalize as much as possible ECCEs for distributed EPDCCH transmissions (at least in terms of REs available for transmitting EPDCCHs) and a need to randomize interference across different cells.
In addition to defining a structure of ECCEs in respective PRB pairs for transmitting distributed EPDCCHs, a first subframe symbol for a PDSCH reception should also be defined. Typically, this is same as a first subframe symbol for EPDCCH reception and can be determined by a UE either by decoding a PCFICH or by higher layer configuration. However, it is possible that a UE may receive a PDSCH scheduled either by a PDCCH (at least in a CSS) or by EPDCCH in which case further considerations are needed for determining a first subframe symbol for PDSCH reception.
An additional issue is the determination by a UE of whether or not to use for PDSCH reception one or more PRB pairs within an RBG when the RBG is indicated for PDSCH reception and the one or more PRB pairs are configured to the UE for transmitting EPDCCHs.
Therefore, there is a need to construct ECCEs for transmitting distributed EPDCCHs for a variable, even number of PRB pairs.
There is another need to improve equivalency among ECCEs in terms of a number of REs for transmitting EPDCCHs and for randomizing EPDCCH interference among different cells.
There is another need for determining a first subframe symbol at a UE for a PDSCH reception that may be scheduled either by a PDCCH or by an EPDCCH.
Finally, there is another need for a UE to determine whether to include in a PDSCH reception a PRB pair configured for an EPDCCH transmission when the PRB pair is included in an RBG indicated for the PDSCH reception.